C/2021 A4 (NEOWISE)
| Parameter |
Value |
| Designation |
C/2021 A4 (NEOWISE) |
| Type |
C |
| Perihelion (tp_cal, UTC) |
2021-03-20T00:53:23.712000Z |
| Orbit solution date (soln_date, UTC) |
2022-03-08T19:43:19Z |
| Obs used |
683 |
| NEO |
- |
| M1 |
13.600 |
| K1 |
28.500 |
| PC |
0.030 |
| Discovery observation (MPC, UTC) |
2021-01-03T22:02:49.632000Z |
| Discovery magnitude (MPC) |
20.00 R |
| Discovery observatory (MPC) |
C51 · WISE |
Osculating Elements
| Parameter |
Value |
| Epoch (epoch_cal, TDB) |
2021-02-16.0000000 |
| e |
0.97324748 |
| q (au) |
1.145476 |
| a (au) |
42.817518 |
| i (deg) |
111.569834 |
| Ω (om, deg) |
307.084762 |
| ω (w, deg) |
204.665566 |
| M (ma, deg) |
359.887300 |
Magnitude
Scatter + Global Linear
$$
M_a = (14.236 + 6.000\cdot 2.5\log_{10} r) + \epsilon - 2.5\log_{10}\Phi(\theta) + 5\log_{10}\Delta
$$
$$
\theta = 180^\circ - \alpha
$$
$$
\Phi(\theta) = \frac{\delta_{90} D(\theta) + 1}{\delta_{90} + 1},\quad D(\theta) = k\cdot \Phi_{HG}(\theta; g_f) + (1-k)\cdot \Phi_{HG}(\theta; g_b)
$$
$$
\Phi_{HG}(\theta; g) = \left(\frac{1+g^2}{1+g^2-2g\cos\theta}\right)^{3/2},\quad \Phi_{HG}(90^\circ)=1
$$
Model Parameters
| Parameter |
Value |
| H0 |
14.236 |
| n |
6.000 |
| gf |
1.000 |
| gb |
-0.500 |
| k |
1.000 |
| δ90 |
10.000 |
| Parameter |
Symbol |
Range |
Description |
| Absolute magnitude at 1 AU from Sun and Earth |
H0 |
-5 to 20 |
Intrinsic brightness of the comet |
| Photometric slope parameter |
n |
2 to 6 |
Rate of brightness change with heliocentric distance |
| Forward scattering asymmetry factor |
gf |
0.8 to 1 |
Degree of forward scattering |
| Backward scattering asymmetry factor |
gb |
-1 to -0.5 |
Degree of backward scattering |
| Partitioning coefficient |
k |
0.8 to 1 |
Ratio between forward/backward scattering contributions |
| Dust-to-gas brightness ratio |
δ90 |
1 to 10 |
Relative contribution of dust scattering to gas emission |
Reference: EPSC-DPS2025-160
Residuals (Piecewise)
$$\epsilon = a \log_{10} r_{\mathrm{au}} + b$$
| Start (UTC) |
End (UTC) |
a |
b |
| 2021-01-03 |
2021-03-19 |
+14.1648 |
-1.643 |
| 2021-03-21 |
2021-07-07 |
-3.2765 |
-1.725 |
log10 r = log10(heliocentric distance / AU), Δ = geocentric distance (AU)
Ephemeris
| Date/Time (UTC) |
RA (J2000) |
Dec (J2000) |
Nearest HIP |
Dist from HIP (arcmin) |
| 2026-02-03 00:00 |
20:52:08.71 |
-22:16:01.2 |
HIP 102930 |
21.00 |
| 2026-02-03 12:00 |
20:52:16.24 |
-22:15:21.0 |
HIP 102930 |
21.62 |
| 2026-02-04 00:00 |
20:52:23.76 |
-22:14:40.9 |
HIP 102930 |
22.39 |
| 2026-02-04 12:00 |
20:52:31.27 |
-22:14:00.8 |
HIP 102930 |
23.28 |
| 2026-02-05 00:00 |
20:52:38.78 |
-22:13:20.9 |
HIP 102930 |
24.28 |
| 2026-02-05 12:00 |
20:52:46.27 |
-22:12:41.0 |
HIP 102948 |
25.16 |
| 2026-02-06 00:00 |
20:52:53.76 |
-22:12:01.3 |
HIP 102948 |
26.00 |
| 2026-02-06 12:00 |
20:53:01.23 |
-22:11:21.6 |
HIP 102948 |
26.95 |
| 2026-02-07 00:00 |
20:53:08.70 |
-22:10:42.0 |
HIP 102948 |
27.99 |
| 2026-02-07 12:00 |
20:53:16.15 |
-22:10:02.6 |
HIP 103274 |
28.49 |
| 2026-02-08 00:00 |
20:53:23.60 |
-22:09:23.2 |
HIP 103274 |
26.72 |
| 2026-02-08 12:00 |
20:53:31.03 |
-22:08:44.0 |
HIP 103274 |
24.96 |
| 2026-02-09 00:00 |
20:53:38.45 |
-22:08:04.8 |
HIP 103274 |
23.22 |
| 2026-02-09 12:00 |
20:53:45.85 |
-22:07:25.8 |
HIP 103274 |
21.50 |
| 2026-02-10 00:00 |
20:53:53.24 |
-22:06:46.8 |
HIP 103274 |
19.80 |
| 2026-02-10 12:00 |
20:54:00.62 |
-22:06:08.0 |
HIP 103274 |
18.12 |
| 2026-02-11 00:00 |
20:54:07.99 |
-22:05:29.3 |
HIP 103274 |
16.49 |
| 2026-02-11 12:00 |
20:54:15.34 |
-22:04:50.7 |
HIP 103274 |
14.90 |
| 2026-02-12 00:00 |
20:54:22.67 |
-22:04:12.3 |
HIP 103274 |
13.37 |
| 2026-02-12 12:00 |
20:54:29.99 |
-22:03:33.9 |
HIP 103274 |
11.92 |
| 2026-02-13 00:00 |
20:54:37.29 |
-22:02:55.7 |
HIP 103274 |
10.59 |
| 2026-02-13 12:00 |
20:54:44.57 |
-22:02:17.6 |
HIP 103274 |
9.42 |
| 2026-02-14 00:00 |
20:54:51.84 |
-22:01:39.6 |
HIP 103274 |
8.47 |
| 2026-02-14 12:00 |
20:54:59.09 |
-22:01:01.7 |
HIP 103274 |
7.84 |
| 2026-02-15 00:00 |
20:55:06.32 |
-22:00:24.0 |
HIP 103274 |
7.58 |
| 2026-02-15 12:00 |
20:55:13.53 |
-21:59:46.4 |
HIP 103274 |
7.74 |
| 2026-02-16 00:00 |
20:55:20.72 |
-21:59:08.9 |
HIP 103274 |
8.28 |
| 2026-02-16 12:00 |
20:55:27.89 |
-21:58:31.6 |
HIP 103274 |
9.14 |
| 2026-02-17 00:00 |
20:55:35.04 |
-21:57:54.4 |
HIP 103274 |
10.24 |
| 2026-02-17 12:00 |
20:55:42.17 |
-21:57:17.4 |
HIP 103274 |
11.50 |
| 2026-02-18 00:00 |
20:55:49.28 |
-21:56:40.5 |
HIP 103274 |
12.87 |
| 2026-02-18 12:00 |
20:55:56.36 |
-21:56:03.7 |
HIP 103398 |
13.75 |
| 2026-02-19 00:00 |
20:56:03.43 |
-21:55:27.1 |
HIP 103398 |
12.44 |
| 2026-02-19 12:00 |
20:56:10.47 |
-21:54:50.6 |
HIP 103398 |
11.24 |
| 2026-02-20 00:00 |
20:56:17.48 |
-21:54:14.3 |
HIP 103398 |
10.20 |
| 2026-02-20 12:00 |
20:56:24.47 |
-21:53:38.1 |
HIP 103398 |
9.38 |
| 2026-02-21 00:00 |
20:56:31.44 |
-21:53:02.1 |
HIP 103398 |
8.82 |
| 2026-02-21 12:00 |
20:56:38.38 |
-21:52:26.2 |
HIP 103398 |
8.58 |
| 2026-02-22 00:00 |
20:56:45.30 |
-21:51:50.5 |
HIP 103398 |
8.68 |
| 2026-02-22 12:00 |
20:56:52.19 |
-21:51:14.9 |
HIP 103398 |
9.10 |
| 2026-02-23 00:00 |
20:56:59.05 |
-21:50:39.6 |
HIP 103398 |
9.80 |
| 2026-02-23 12:00 |
20:57:05.89 |
-21:50:04.3 |
HIP 103398 |
10.72 |
| 2026-02-24 00:00 |
20:57:12.70 |
-21:49:29.3 |
HIP 103398 |
11.80 |